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Jets, high-p T hadrons and prompt photons QM2011 student lecture 22 May 2011 Marco van Leeuwen, Utrecht University
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2 Hard processes in QCD Hard process: scale Q >> QCD Hard scattering High-p T parton(photon) Q~p T Heavy flavour production m >> QCD Cross section calculation can be split into Hard part: perturbative matrix element Soft part: parton density (PDF), fragmentation (FF) Soft parts, PDF, FF are universal: independent of hard process QM interference between hard and soft suppressed (by Q 2 / 2 ‘Higher Twist’) Factorization parton densitymatrix elementFF
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3 Seeing quarks and gluons In high-energy collisions, observe traces of quarks, gluons (‘jets’)
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4 Fragmentation and parton showers large Q 2 Q ~ m H ~ QCD FF Analytical calculations: Fragmentation Function D(z, ) z=p h /E jet Only longitudinal dynamics High-energy parton (from hard scattering) Hadrons MC event generators implement ‘parton showers’ Longitudinal and transverse dynamics
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5 Jet Quenching 1)How is does the medium modify parton fragmentation? Energy-loss: reduced energy of leading hadron – enhancement of yield at low p T ? Broadening of shower? Path-length dependence Quark-gluon differences Final stage of fragmentation outside medium? 2)What does this tell us about the medium ? Density Nature of scattering centers? (elastic vs radiative; mass of scatt. centers) Time-evolution? High-energy parton (from hard scattering) Hadrons
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6 Medium-induced radition If < f, multiple scatterings add coherently Zapp, QM09 L c = f,max propagating parton radiated gluon Landau-Pomeranchuk-Migdal effect Formation time important Radiation sees length ~ f at once Energy loss depends on density: and nature of scattering centers (scattering cross section) Transport coefficient
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7 Testing volume (N coll ) scaling in Au+Au PHENIX Direct spectra Scaled by N coll PHENIX, PRL 94, 232301 Direct in A+A scales with N coll Centrality A+A initial state is incoherent superposition of p+p for hard probes
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8 0 R AA – high-p T suppression Hard partons lose energy in the hot matter : no interactions Hadrons: energy loss R AA = 1 R AA < 1 0 : R AA ≈ 0.2 : R AA = 1
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9 Two extreme scenarios p+p Au+Au pTpT 1/N bin d 2 N/d 2 p T Scenario I P( E) = ( E 0 ) ‘Energy loss’ Shifts spectrum to left Scenario II P( E) = a (0) + b (E) ‘Absorption’ Downward shift (or how P( E) says it all) P( E) encodes the full energy loss process R AA not sensitive to energy loss distribution, details of mechanism
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10 Four theory approaches Multiple-soft scattering (ASW-BDMPS) –Full interference (vacuum-medium + LPM) –Approximate scattering potential Opacity expansion (GLV/WHDG) –Interference terms order-by-order (first order default) –Dipole scattering potential 1/q 4 Higher Twist –Like GLV, but with fragmentation function evolution Hard Thermal Loop (AMY) –Most realistic medium –LPM interference fully treated –No finite-length effects (no L 2 dependence)
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11 Energy loss spectrum Brick L = 2 fm, E/E = 0.2 E = 10 GeV Typical examples with fixed L E/E> = 0.2 R 8 ~ R AA = 0.2 Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to E/E = 1) Theory expectation: mix of partial transmission+continuous energy loss – Can we see this in experiment?
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12 Geometry Density profile Profile at ~ form known Density along parton path Longitudinal expansion dilutes medium Important effect Space-time evolution is taken into account in modeling
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13 Determining Bass et al, PRC79, 024901 ASW: HT: AMY: Large density: AMY: T ~ 400 MeV Transverse kick: qL ~ 10-20 GeV All formalisms can match R AA, but large differences in medium density At RHIC: E large compared to E, differential measurements difficult After long discussions, it turns out that these differences are mostly due to uncontrolled approximations in the calculations Best guess: the truth is somewhere in-between
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14 R AA at LHC ALICE PHENIX R AA at LHC: increase with p T first sign of sensitivity to P( E) ALICE, PLB 696, 30 Larger ‘dynamic range’ at LHC very important – stay tuned
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15 R AA RHIC and LHC II Overlaying the two results: PHENIX 0 and ALICE h ± p T -dependence not too different… N.B.: Large uncertainties in RHIC result at high p T
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16 Path length dependence: R AA vs L PHENIX, PRC 76, 034904 In Plane Out of Plane 3<p T <5 GeV/c R AA as function of angle with reaction plane Suppression depends on angle, path length Relation between RAA( ) and v 2 :
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17 Path length dependence and v 2 PHENIX PRL105, 142301 v 2 at high p T due to energy loss Most calculations give too small effect Path length dependence stronger than expected? Depends strongly on geometry – stay tuned
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18 Dihadron correlations associated trigger 8 < p T trig < 15 GeV p T assoc > 3 GeV Use di-hadron correlations to probe the jet-structure in p+p, d+Au Near side Away side and Au+Au Combinatorial background
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19 p T assoc > 3 GeV p T assoc > 6 GeV d+Au Au+Au 20-40% Au+Au 0-5% Suppression of away-side yield in Au+Au collisions: energy loss High-p T hadron production in Au+Au dominated by (di-)jet fragmentation Di-hadrons at high-p T : recoil suppression
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20 Dihadron yield suppression Away-side: Suppressed by factor 4-5 large energy loss Near side Away side STAR PRL 95, 152301 8 < p T,trig < 15 GeV Yield of additional particles in the jet Yield in balancing jet, after energy loss Near side: No modification Fragmentation outside medium? Near side associated trigger Away side associated trigger
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21 Path length II: ‘surface bias’ Near side trigger, biases to small E-loss Away-side large L Away-side suppression I AA samples longer path-lengths than inclusives R AA
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22 L scaling: elastic vs radiative T. Renk, PRC76, 064905 R AA : input to fix density Radiative scenario fits data; elastic scenarios underestimate suppression Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions Confirms L 2 dependence radiative loss dominates
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23 Intermediate p T Enhanced baryon/meson ratio –Hadronisation by coalescence? Enhanced near-side yield at large ‘ridge’ –Triangular flow? Away-side double-peak structure –Mach cone? –Triangular flow? So far, focused on high-p T Where factorisation may hold p T > 1-4 GeV Some other ‘puzzling’ (i.e. not dominated by jet fragmention+energy loss) observations at intermediate p T :
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24 Baryon excess STAR Preliminary B. Mohanty (STAR), QM08 High p T : Au+Au similar to p+p Fragmentation dominates Baryon/meson = 0.2-0.5 Intermediate p T, 2 – 6 GeV Large baryon/meson ration in Au+Au
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25 Hadronisation through coalescence fragmenting parton: p h = z p, z<1 recombining partons: p 1 +p 2 =p h Fries, Muller et al Hwa, Yang et al Meson p T =2p T,parton Recombination of thermal (‘bulk’) partons produces baryons at larger p T Recombination enhances baryon/meson ratio Hot matter Baryon p T =3p T,parton R. Belmont, QM09 Note also: v 2 scaling
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26 Near-side Ridge 3 < p t,trig < 4 GeV/c Au+Au 0-10% STAR preliminary associated trigger `Ridge’: associated yield at large , small Ridge softer than jet – medium response probably v 3 J. Putschke et al, QM06 Weak dependence of ridge yield on p T,trig Relative contribution reduces with p T,trig 4 < p t,trig < 6 GeV/c Au+Au 0-10% STAR preliminary Jet-like peak p t,assoc. > 2 GeV/c
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27 0-12% 4.0 < p T trig < 6.0 GeV/c 6.0 < p T trig < 10.0 GeV/c 3.0 < p T trig < 4.0 GeV/c Preliminary Au+Au 0-12% 1.3 < p T assoc < 1.8 GeV/c Low p T trig : broad shape, two peaks High p T trig : broad shape, single peak Away-side shapes Fragmentation becomes ‘cleaner’ as p T trig goes up Suggests kinematic effect? M. Horner, M. van Leeuwen, et al
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28 v 3, triangular flow Alver and Roland, PRC81, 054905 Participant fluctuations lead to triangular component of initial state anisotropy This may well be the underlying mechanism for both ‘ridge’ and ‘Mach cone’
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29 Jet reconstruction Single, di-hadrons: focus on a few fragments of the shower No information about initial parton energy in each event Jet finding: sum up fragments in a ‘jet cone’ Main idea: recover radiated energy – determine energy of initial parton Feasibility depends on background fluctuations, angular broadening of jets Need: tracking or Hadron Calorimeter and EMCal ( 0 )
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30 Generic expectations from energy loss Longitudinal modification: –out-of-cone energy lost, suppression of yield, di-jet energy imbalance –in-cone softening of fragmentation Transverse modification –out-of-cone increase acoplanarity k T –in-cone broadening of jet-profile kT~kT~ E jet fragmentation after energy loss?
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31 Modified fragmentation functions A. Majumder, MvL, arXiv:1002.2206 z = p h|| /E jet Fragmentation function ratio =ln(E Jet /p hadron ) z Borghini and Wiedemann Fragmentation function ‘Hump-backed plateau’ Expect softening of fragmentations: fewer fragments at high p T, more at low p T
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32 Jet shapes Energy distribution in sub-jets Energy loss changes radial distribution of energy Several ‘new’ observables considered Discussion: sensitivity viability … ongoing q-Pythia, Eur Phys J C 63, 679
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33 Fixing the parton energy with -jet events T. Renk, PRC74, 034906 -jet: know jet energy sensitive to P( E) R AA insensitive to P( E) Nuclear modification factor Away-side spectra in -jet E = 15 GeV Away-side spectra for -jet are sensitive to P( E) Input energy loss distribution
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34 I AA (z T ) = D AA (z T ) D pp (z T ) Direct- recoil suppression Large suppression for away-side: factor 3-5 Reasonable agreement with model predictions 8 < E T, < 16 GeVSTAR, arXiv:0912.1871 NB: gamma p T = jet p T still not very large
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35 Thermal photons PHENIX, PRL 104, 132301 Idea: hot quark-gluon matter radiates photons which escape Difficult measurement: Large background 0 → Thermal photons at low p T Excess of photons seen at RHIC
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36 Jet reconstruction algorithms Two categories of jet algorithms: Sequential recombination k T, anti-k T, Durham –Define distance measure, e.g. d ij = min(p Ti,p Tj )*R ij –Cluster closest Cone –Draw Cone radius R around starting point –Iterate until stable , jet = particles For a complete discussion, see: http://www.lpthe.jussieu.fr/~salam/teaching/PhD-courses.html Sum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectors Jet is an object defined by jet algorithm If parameters are right, may approximate parton
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37 Collinear and infrared safety Illustration by G. Salam Jets should not be sensitive to soft effects (hadronisation and E-loss) -Collinear safe -Infrared safe
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38 Jet finding in heavy ion events η p t per grid cell [GeV] STAR preliminary ~ 21 GeV FastJet:Cacciari, Salam and Soyez; arXiv: 0802.1188 http://rhig.physics.yale.edu/~putschke/Ahijf/A_Heavy_Ion_Jet-Finder.html Jets clearly visible in heavy ion events at RHIC Use different algorithms to estimate systematic uncertainties: Cone-type algorithms simple cone, iterative cone, infrared safe SISCone Sequential recombination algorithms k T, Cambridge, inverse k T Combinatorial background Needs to be subtracted
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39 Jet finding with background By definition: all particles end up in a jet With background: all - space filled with jets Many of these jets are ‘background jets’
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40 Background subtraction STAR Preliminary multiplicity Background energy density (GeV) Background density at RHIC: 60-100 GeV Strong dependence on centrality Fluctuations remain after subtraction: RMS up to 10 GeV
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41 Jets at LHC LHC: jet energies up to ~200 GeV in Pb+Pb from 1 ‘short’ run Large energy asymmetry observed for central events
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42 Jets at LHC Centrality ATLAS, arXiv:1011.6182 (PRL) Jet-energy asymmetry Large asymmetry seen for central events N.B. only measures reconstructed di-jets Does not show ‘lost’ jets Large effect on recoil: qualitatively consistent with RHIC jet I AA Energy losses: tens of GeV, ~ expected from BDMPS, GLV etc beyond kinematic reach at RHIC
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43 Jets at LHC CMS, arXiv:1102.1957 CMS sees similar asymmetries
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44 Summary Hard processes can be used to probe quark-gluon matter So far: main focus on energy loss of (leading) high-p T hadrons –Integrates over initial energy, energy-loss For radiative energy loss expect E L 2 –Di-hadron recoil suppression confirms this –Azimuthal dependence of energy loss (v 2 at high p T ) not yet quantitatively understood Future directions: better handle on initial parton energy –Jet finding – -jet
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45 Extra slides
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46 Hard probes of QCD matter Use the strength of pQCD to explore QCD matter Hard-scatterings produce ‘quasi-free’ partons Initial-state production known from pQCD Probe medium through energy loss Heavy-ion collisions produce ‘quasi-thermal’ QCD matter Dominated by soft partons p ~ T ~ 100-300 MeV Sensitive to medium density, transport properties
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47 Toy model R AA This is a cartoon! Hadronic, not partonic energy loss No quark-gluon difference Energy loss not probabilistic P( E) Ball-park numbers: E/E ≈ 0.2, or E ≈ 2 GeV for central collisions at RHIC 0 spectra Nuclear modification factor PHENIX, PRD 76, 051106, arXiv:0801.4020 Note: slope of ‘input’ spectrum changes with p T : use experimental reach to exploit this
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48 Collinear safety Note also: detector effects, such as splitting clusters in calorimeter ( 0 decay) Illustration by G. Salam
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49 Infrared safety Infrared safety also implies robustness against soft background in heavy ion collisions Illustration by G. Salam
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50 Shockwave/Mach Cone Gyulassy et al arXiv:0807.2235 T. Renk, J. Ruppert B. Betz, QM09, PRC79, 034902 Mach-cone/shockwave in the QGP? Exciting possibility! Are more mundane possibilities ruled out? – Not clear yet Proves that QGP is really ‘bulk matter’ Measure speed of sound?
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51 Jet broadening II Diffuse broadening Hard radiation/splitting Qualitatively, two different possible scenarios Different measurements: -R(0.2/0.4) -Transverse jet profile May have different sensitivities Interesting idea: sub-jet structure; so far no studies available Radiated energy ‘uniformly’ distributed Radiated energy directional
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52 Determining the medium density PQM (Loizides, Dainese, Paic), Multiple soft-scattering approx (Armesto, Salgado, Wiedemann) Realistic geometry GLV (Gyulassy, Levai, Vitev), Opacity expansion (L/ ), Average path length WHDG (Wicks, Horowitz, Djordjevic, Gyulassy) GLV + realistic geometry ZOWW (Zhang, Owens, Wang, Wang) Medium-enhanced power corrections (higher twist) Hard sphere geometry AMY (Arnold, Moore, Yaffe) Finite temperature effective field theory (Hard Thermal Loops) For each model: 1.Vary parameter and predict R AA 2.Minimize 2 wrt data Models have different but ~equivalent parameters: Transport coeff. Gluon density dN g /dy Typical energy loss per L: 0 Coupling constant S PHENIX, arXiv:0801.1665, J. Nagle WWND08
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53 Medium density from R AA PQM = 13.2 GeV 2 /fm +2.1 - 3.2 ^ GLV dN g /dy = 1400 +270 - 150 WHDG dN g /dy = 1400 +200 - 375 ZOWW 0 = 1.9 GeV/fm +0.2 - 0.5 AMY s = 0.280 +0.016 - 0.012 Data constrain model parameters to 10-20% Method extracts medium density given the model/calculation Theory uncertainties need to be further evaluated e.g. comparing different formalisms, varying geometry But models use different medium parameters – How to compare the results?
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54 Some pocket formula results Large differences between models GLV/WHDG: dN g /dy = 1400 T( 0 ) = 366 MeV PQM: (parton average) T = 1016 MeV AMY: T fixed by hydro (~400 MeV), s = 0.297 – After long discussions, it turns out that most of these differences are mostly due to uncontrolled approximations in the calculations Best guess: the truth is somewhere in-between
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55 Comparing single- and di-hadron results Armesto, Cacciari, Salgado et al. R AA and I AA fit with similar density Calculation uses LPM-effect, L 2 dependence
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56 QCD : For SU(3) : N c = 3 C A = 3, C F = 4/3 C F ~ strength of a gluon coupling to a quark C A ~ strength of the gluon self coupling T F ~ strength of gluon splitting into a quark pair C A /C F =9/4 Color factors Color factors measured at LEP Expect gluons radiate ~ twice more energy than quarks
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57 PYTHIA (by Adam Kocoloski) gg qq gq Subprocesses and quark vs gluon p+pbar dominantly from gluon fragmentation
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58 PRL 97, 152301 (2006) STAR Preliminary, QM08 Curves: X-N. Wang et al PRC70(2004) 031901 Baryon & meson NMF STAR Preliminary Comparing quark and gluon suppression Protons less suppressed than pions, not more No sign of large gluon energy loss
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59 Quark vs gluon suppression + renk plot WHDG Renk and Eskola, PRC76,027901 GLV formalism BDMPS formalism Quark/gluon difference larger in GLV than BDMPS (because of cut-off effects E < E jet ?) ~10% baryons from quarks, so baryon/meson effect smaller than gluon/quark Are baryon fragmentation functions under control? Conclusion for now: some homework to do...
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60 The fine-print: background 3.0 < p T trig < 4.0 GeV/c 1.3 < p T assoc < 1.8 GeV/c High p T : background <~ signal 8 < p T trig < 15 GeV p T assoc > 3 GeV Low p T : background >> signal v 2 modulated background v 2trig * v 2assoc ~ few per cent Background normalisation: Zero Yield At Minimum N.B. no signal-free region at low p T
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61 v 3 in Hydro Schenke, Jeon, Gale, PRL 106, 042301 Evolution of initial state spatial anisotropy depends on viscosity
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62 v 3 vs eps v 3 from AMPT Schenke and Jeon v 3 from Hydrodynamics Alver and Roland Initial triangular anysotropy gives rise to v 3 in both parton cascade and hydrodynamics v 3 can be the underlying mechanism for both ‘ridge’ and ‘Mach cone’
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63 Modelling azimuthal dependence A. Majumder, PRC75, 021901 R AA p T (GeV) R AA R AA vs reaction plane sensitive to geometry model
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64 pQCD illustrated CDF, PRD75, 092006 jet spectrum ~ parton spectrum fragmentation
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65 R AA vs reaction plane angle Azimuthal modulation, path length dependence largest in ASW-BDMPS Data prefer ASW-BDMPS But why? – No clear answer yet Majumder, van Leeuwen, arXiv:1002.2206
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66 Interpreting di-hadron measurements Scenario I: Some lose all, Some lose nothing Scenario II: All lose something Di-hadron measurement: Away-side yield is (semi-)inclusive, so does not measure fluctuations of energy loss Multi-hadron measurements potentially more sensitive All is encoded in energy loss distribution P( E)
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67 A closer look at azimuthal peak shapes No away-side broadening: -No induced radiation -No acoplanrity (‘multiple-scattering’) Induced acoplanarity (BDMPS): Vitev, hep-ph/0501225 Broadening due to fragments of induced radiation 8 < p T (trig) < 15 GeV/c p T (assoc)>6 GeV
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68 Fragmentation functions Qualitatively: Fragmentation functions sensitive to P( E) Distinguish GLV from BDMPS?
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69 Parton energy loss and R AA modeling Qualitatively: `known’ from e + e - known pQCDxPDF extract Parton spectrum Fragmentation (function) Energy loss distribution medium effect Medium effect P( E) is only part of the story Parton spectrum and fragmentation function are steep non-trivial relation between R AA and P( E)
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